But according to new research, which asked 66 men and women to swab their navels with a sterile Q-tip, the skin in study participants' belly buttons also contained an average of 67 different species of bacteria.

Why belly buttons? "It was a fun way to reach out to the public and teach them about the ecology and evolution of everyday life," says Rob Dunn, PhD, an associate professor of biology at North Carolina State University in Raleigh, who led the study. In other words, the navel is a novel, attention-getting device to study science.

He says the belly button is a fun habitat filled with living organisms that we don't know much about. It's less exposed and gets washed less often than other areas of skin, so the bacterial community in the umbilicus is less disturbed.

In all, they found 2,368 different species of bacteria, which is a heck of a lot of biological diversity.

"We got many more species of bacteria than we expected," says Dunn. But most of those bacterial species were rare ones found in just a few people's belly buttons.

Only about 8 bacterial types occurred in more than 70% of all the people screened.

Those common kinds included species such as Staphylococci, which Dunn says is like your skin's standing army defending it from bad germs. Other frequent microbes were a species of Bacillus, a type that gives stinky feet their odor and may be protecting the body from fungi, and Micrococcus, a hardy bacteria found deep in the navel that can survive without oxygen.

The more common species of bacteria seem to be very predictable, Dunn explains. "They were more frequent and abundant on more people, and more common than we expected," he points out.

Dunn suggests that if scientists can get a handle on those common ones, they will know a lot more about what's going on with skin bacteria. For example, they might understand which ones are really good for the skin and which ones are bad. Or how the bacteria interact with one another or with the immune system.

Two samples contained an extremely rare type of archaea, a single-cell organism never previously found on human skin. One of these samples came from a man who self-reported that he had not bathed or showered for several years -- yikes!

Researchers also collected information from study participants on their age, gender, ethnicity, where they grew up, if they are pet owners (who may get more bacteria on their skin if their pooch or cat frequently lick them), and even if their belly button was an innie or an outie. So far, none of this data has been linked to the types of bacterial species found in someone's umbilicus.